Expansion of Freshwater Harmful Cyanobacteria in Brackish Waters: Salt Stress Responses, Toxigenic Dynamics, and Ecological Risks along the Freshwater-to-Marine Continuum

Abstract In recent years, freshwater harmful cyanobacterial blooms (CyanoHABs) and their associated cyanotoxins have been increasingly detected in estuaries and coastal lagoons, indicating that some freshwater taxa can survive in low-to-moderate salinity and may maintain their toxigenic potential in brackish environments. These observations highlight the need to understand the distribution, persistence, and ecological risks of freshwater harmful cyanobacteria across connected aquatic systems. This review synthesizes global field and mechanistic evidence on the expansion of freshwater harmful cyanobacteria along the freshwater–to–marine continuum. Hydrodynamic processes facilitate downstream transport, while salinity gradients regulate cyanobacterial survival and physiological responses. Salt stress induces cellular damage but also triggers adaptive physiological responses supporting survival. The persistence of freshwater harmful cyanobacteria in these systems is shaped by both species- and strain-level differences in salt tolerance, as well as physiological conditioning and environmental conditions experienced during transport. In addition, salinity influences cyanotoxin production, release, partitioning between particulate and dissolved fractions, and environmental persistence, thereby influencing ecological risks in brackish waters. Overall, this review synthesizes cyanobacterial adaptation and cyanotoxin dynamics along the freshwater–to–marine continuum and emphasizes integrated monitoring and modeling to support risk assessment and management of CyanoHABs.